Your helmet / headlamp functions can be accessed by pressing the Number '1' key. This will bring up a panel showing the headlamps's settings, as follows:

Light On / Off - Toggle the helmet's forward facing light on and off (Keyboard shortcut - L)

The colour of an (unworn) headlamp can be changed using Spray Paint. However, this chnages the colour of the lenses (without any effect on the light genertaed), rather than changing the colourtr of the headband as in the case of Night Vision Goggles.